Office visits are the bread and butter of many physicians' practices. Medicare pays for more than 200 million of them a year, often to deal with routine problems like colds or high blood pressure. Most require relatively modest amounts of a doctor's time or medical know-how. Not so for Michigan obstetrician-gynecologist Obioma Agomuoh. He charged for the most complex — and expensive — office visits for virtually every one of his 201 Medicare patients in 2012, his billings show. In fact, Medicare paid Agomuoh for an average of eight such visits per patient that year, a staggering number compared with his peers.

Doctors and other health providers nationwide charged the top rate in 2012 for just 4 percent of office visits for patients they had seen before. But Agomuoh was one of more than 1,800 health professionals nationwide who billed Medicare for the most expensive type of office visits at least 90 percent of the time that year, a ProPublica analysis of newly released Medicare data found.

Dr. John Im, who runs an urgent care center in the Villages, the large retirement community near Ocala, charged the program at that level for all 2,376 visits by his established patients. Kaveh Farhoomand, an Oceanside, Calif., internist facing disciplinary charges from his state medical board, collected the highest rate to see almost all of his 301 Medicare patients an average of seven times each.

By exposing such massive variations in how doctors bill the nation's health program for seniors and the disabled, experts said, ProPublica's analysis shows Medicare could — and should — be doing far more to use its own data to sniff out cost-inflating errors and fraud.

"I think this is a smoking gun," said Dr. Robert Berenson, a former senior Medicare official who is now a fellow at the Urban Institute, a Washington, D.C., think tank. "Who's asleep at the switch here?"

The Centers for Medicare and Medicaid Services, which runs Medicare, declined an interview request and said in a statement that it could not comment on ProPublica's analysis because it had not seen it. The agency declined to discuss individual providers but said their data may not take into account money collected by a provider and subsequently returned to CMS, or payments that "may have been withheld after claims were already processed but prior to release to the provider."

"CMS is working to ensure that physicians and health care providers appropriately bill" for office visits, also known as evaluation and management (E&M) services, the agency said.

American Medical Association president Dr. Ardis Dee Hoven cautioned that billing data can be misleading without considering further details about doctors' practices. Even those who handle medical billing professionally sometimes disagree about the right way to classify a visit.

Agomuoh, Im and Farhoomand insist that they treat older, sicker or more difficult patients than their peers. Agomuoh also suggested that the Medicare data contained errors; the agency stands behind it.

Individually, office visits for established patients cost taxpayers little, ranging from an average of $14 for the simplest cases to more than $100 for the most extensive. But collectively, they add up. Medicare shelled out more than $12 billion for them in 2012.

In April, Medicare released data showing 2012 payments for outpatient services, and for the first time specified how much money went to individual health providers. The data allow consumers to look at which procedures doctors are performing and how frequently, and how their billings compare with those of their peers.

Doctors or their staffs determine how to bill for routine office visits for established patients (those the provider has seen at least once before) based on a variety of factors, including the thoroughness of the review of a patient's medical history, the comprehensiveness of the physical exam, and the complexity of medical decision-making involved. The AMA's coding system gives them five options.

An uncomplicated visit, typically of short duration, should be coded a "1"; a visit that involves more intense examination and often consumes more time should be coded a "5." The most common code for visits is in the middle, a "3."

ProPublica focused its analysis on the 329,500 physicians and other providers who charged for at least 100 office visits for established patients (Medicare did not release data on services that a provider performed on fewer than 11 patients.)

We found that while most providers had a tiny percentage of level 5 cases, more than 1,200 billed exclusively at the highest level. Another 600 did it more than 90 percent of the time. About 20,000 health professionals billed only at levels 4 or 5.

"I can't see a situation where every visit would be a level 5, especially on an established patient," said Cyndee Weston, executive director of the American Medical Billing Association, an industry trade group. "I was trying to talk myself into it, but I just can't see it."

A May 2012 report from the U.S. Department of Health and Human Services' inspector general found that doctors are choosing higher codes more often for evaluation and management services, the broad category that includes office visits.

Agomuoh, the Michigan ob/gyn, stood out for reasons beyond his office visits. Medicare paid for an average of eight wheezing evaluations for each of his patients in 2012, at $50 a pop. Almost all of his patients received multiple ultrasounds of arteries in the legs (at $149 per test) and arms (at $144 per test), records show. Most of his peers rarely, if ever, performed these services.

Agomuoh said his billings reflect that many of his patients have asthma, chronic obstructive pulmonary disease and drug addictions.